The manufacture of products from heat curable or thermosetting polymeric materials utilizing continuous forming procedures, such as extrusion processes, requires ancillary means or steps to achieve a continuous and effective heat induced curing of the formed product in concert with the operation of the extruder and its rate of output. Moreover, the usual means or steps for heat curing a continuously produced product are frequently cumbersome, expensive, and sometimes impede the overall manufacturing process or affect product quality.
For example, elongated steam chambers, typically comprising a long length of pipe supplied with steam, are commonly employed in tandem with an extruder apparatus to raise the temperature of heat curable extruded products up to the level of their curing condition. A product continuously emanating from the forming extruder is passed through the elongated steam chamber and while moving therethrough the heat curable product is heated to curing conditions by contact with the steam atmosphere. The attainment of curing temperatures or conditions with such continuous curing systems depends, of course, upon several significant factors, including especially the temperature of the steam or of the chamber atmosphere and the residence time of exposure of the heat curable material thereto, as well as the mass and the heat transferring or conductivity properties of the material to be heated while in passing contact with the heat source. Thus, to achieve increased rates of production output in such continuous manufacturing operations for heat cured products, either the temperature of the steam or other heating medium within the curing chamber, or the residence time of the product continuously moving through the curing chamber and in contact with the steam or other heating medium, or both, must be increased to effect the cure in the faster produced and moving materials.
However, because of the well known temperature-pressure characteristics of steam, any increase in steam temperatures is inherently accomplished by a disproportionally greater increase in pressure. The extensive increases in steam pressures due to moderate increases in its temperature to accelerate curing rates or the conditions for curing, require costly and massive confining measures and complex apparatus to generate, transmit or handle, and operate high pressure curing chambers for the continuous passage of products therethrough. Accordingly, rather than increase steam temperatures and incur excessive steam pressures, steam curing chambers have been extended further and further in length as the production capabilities of modern extrusion apparatus have been increased and manufacturing costs have pressed the need for greater output in order to provide sufficient residence time therein for contact with the steam to reach curing temperatures through the overall mass of the product continuously and rapidly moving therethrough.
Extensions of the length of heat curing chambers commensurate with increased production output capabilities or improved extruders to attain the necessary product residence time to reach curing temperature conditions penetrating the depth of the mass of the heat curable material for effective complete cures, creates inconvenient and excessive factory or floor space requirements and costs, as well as problems in handling and transporting the ware through chambers of great length.
The obvious shortcomings of such systems have prompted efforts to develop more effective and less costly apparatus and systems for effecting a rapid and uniformly complete cure in continuously produced heat curable products. For example, U.S. Pat. Nos. 3,513,228 and 3,635,621 each proposes the application of radiant heat to heat curable material or products thereof within a pressurized atmosphere of inert gas, and U.S. Pat. No. 3,645,556 discloses the application of heat through the use of a stagnant body of nitrogen gas under super-atmospheric pressure as a heat transfer medium. Nevertheless, as is apparent, these systems are also complex and require elaborate apparatus.
Another approach to this problem of achieving a rapid cure in products continuously produced with extruding devices, has been to apply heating means in or adjacent to the forming die or outlet passage of extruder apparatus, such as shown in U.S. Pat. Nos. 2,547,151 and 2,972,780.
Moreover, the use of steam under high pressures may produce adverse effects upon some polymeric compositions, or may impede their use in some classes of products or service. For example, high pressure steam drives moisture into polymeric materials or compositions and such a moisture containing condition is especially detrimental in electrical insulations and, accordingly, the moisture must be removed prior to use in electrical applications as noted in U.S. Pat. No. 3,054,142. Thus electrical products such as wires and cables which are insulated with high pressure steam cured polymeric materials require a purging operation such as a heat treatment to drive out entrained moisture from the steam. This adds considerably to the complexity and costs of manufacture.
Additionally polymeric materials are relatively poor conductors of heat, and prior art systems or techniques entail applying an external heat source or heat transfer medium to the exterior of the body of the polymeric material whereby the residence time of exposure to the heating medium or source must be prolonged to achieve curing condition temperatures and their penetration throughout the mass of the body of polymeric material for a uniform cure therethrough. Moreover, due to the relatively poor thermal conductivities of polymeric materials, the application or relatively high temperatures in order to accelerate the curing rate can result in thermal deterioration or damage of the outer portions of the materials before the inner portions are raised to curing conditions.